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thermal meltout. During thermal meltout, the ice cover deteriorates through warming and the <br />absorption of solar radiation and melts in place, with no increase in flow and little or no ice <br />movement. At the other extreme is the more complex and less understood mechanical breakup. <br />Mechanical breakup requires no thermal deterioration of the ice cover but rather results from an <br />increase in flow entering the river (e.g., runoff from melting snow). The increase in flow induces <br />stresses in the cover, and the stresses in turn cause cracks and the ultimate fragmentation of the <br />ice cover into pieces that are transported by the channel flow. Ice jams occur at locations where <br />the ice fragments stop; severe and sudden flooding can result when these ice jams form or when <br />they release. Most river ice breakups actually fall somewhere in between the extremes of <br />thermal meltout and mechanical breakup because breakup usually occurs during warming periods <br />when the ice cover strength deteriorates to some degree and the flow entering the river increases <br />due to snow melt or precipitation. As a general rule, the closer that a breakup is to being a <br />mechanical breakup, the more dramatic and dangerous it is because of the sudden increase in <br />flow and the large volume of fragmented ice produced. <br />-5-